The present invention relates to optically retiming an optical data signal, and an optical retimer for performing the retiming. One of the advantages of using the optical retimer of the present invention for performing retiming of an optical data signal is that it improves the integrity of the retimed optical signal relative to prior art techniques that convert the optical signal from the optical domain to the electrical domain, perform retiming in the electrical domain, and then re-convert the electrical signal to a retimed optical signal. In other words, the present invention eliminates, or at least decreases, the potential for errors (e.g., jitter) existing in the retimed optical signal. This is particularly important at high data rates, greater than a few billion bits per second, for example, where retiming in the electrical domain is severely limited by the speed of today's integrated circuit processes.
In optical fiber communications links, an optical source, typically a laser, is modulated between two optical power levels, the lower level indicating a logical 0, and the upper level indicating an optical 1. This optical source, e.g., the laser, is modulated at a specific, fixed bit rate to produce an optical data signal comprising logical 1s and 0s, which propagates along an optical fiber. As the optical data signal travels over a significant distance of the optical fiber, and passes through multiple optical-electrical-optical regeneration stages, the integrity of the optical data signal can degrade, thereby causing the “eye” to close. The “eye” corresponds to the integrity of the signal. The eye can be closed due to either loss of signal integrity in the horizontal direction caused by jitter in the rising and falling edges of the bit, or due to loss of integrity in the vertical direction as a result the signal losing its strength or noise in the signal.
In order to restore the integrity of a degrading signal, and “open the eye”, an electronic retimer is used in the optical-electrical-optical regeneration stage. In an electrical retimer, the optical data signal is converted into an electrical data signal and an electrical clock signal that is synchronized to the data bit rate periodically causes the converted electrical data signal to be sampled over a small time window. This time window corresponds to a fraction of the corresponding bit period and is typically referred to as the “sampling aperture”. The electrical retimer makes a decision of the logic level of the signal based on the average signal level within the sampling aperture, and then outputs a signal that is held at the determined logic level (i.e., 0 or 1) for an entire bit period. This removes the jitter from the signal and also regenerates the original square waveform, thereby increasing the signal integrity in both the vertical and the horizontal directions.
In order to use an electrical retimer in an optical data link, the optical signal must first be detected by a photodetector, converted from an optical signal to an electrical signal, retimed in the electrical domain and then reconverted back to an optical signal. One of the problems associated with performing retiming in this manner is that retiming in the electrical domain is typically performed by an integrated circuit (IC), and increases in the data bit rate (and the corresponding decreases in the sampling aperture) may present problems in producing an IC that can perform the retiming operations at the necessary speeds.
Accordingly, a need exists for an optical retimer that avoids the need for optical-to-electrical and electrical-to-optical conversion and that is capable of optically retiming an optical data signal to obtain a retimed optical data signal with high integrity.